1. Field of the Invention
The present invention relates to a method of fabricating a group II-VI compound semiconductor thin film, and a group II-VI compound semiconductor device. More particularly, the present invention relates to a semiconductor device having a semiconductor layer made of a material of ZnCdSSe type.
2. Description of the Related Art
A blue-light-emitting diode and a semiconductor laser emitting blue light are extremely useful for light sources in various opto-electronics apparatus such as a full-color display device, a high-density information processing device, and a photo-chemical reacting processing device. Thus, in recent years, such a blue-light-emitting diode and a semiconductor laser emitting blue light have been intensively investigated.
In a conventional group II-VI compound semiconductor device, a single-crystal substrate made of a group III-V semiconductor such as GaAs, or GaP, or a group II-VI single-crystal semiconductor such as ZnS, or ZnSe is used. On the substrate, a single-crystal semiconductor layer having a multi-layer structure made of a group II-VI compound semiconductor is deposited by molecular-beam epitaxy (MBE), metalorganic chemical vapor deposition (MOCVD), or the like. Since a (100) single-crystal substrate is inexpensive and easily available, it is most widely used for the substrate on which the group II-VI compound semiconductor single-crystal thin film is grown (for example, M. A. Haase et al., Appl. Phys. Lett. 59, 1272 (1991)).
A substrate having a (111)A plane and a substrate having a (111)B plane are also easily available, so that the growth of a group II-VI compound semiconductor thin film on such a substrate has been investigated. However, the group II-VI compound semiconductor thin film is grown on the substrate having a (111)A plane or the substrate having a (111)B plane in a twin-crystal state. This means that a good group II-VI compound semiconductor thin film cannot be obtained. Hitherto, the growth of such a semiconductor thin film on substrates having other orientation planes than (100), (111)A, and (111)B planes have not been investigated.
FIG. 16 shows a cross-sectional construction of a conventional current injection type semiconductor laser device 720 having a group II-VI compound semiconductor layer. On an n-type GaAs (100) substrate 701, a buffer layer 702 of n-type GaAs, a contact layer 703 of an n.sup.+ -type ZnSe, a cladding layer 704 of n-type ZnS.sub.0.07 Se.sub.0.93, a light guiding layer 705 of n-type ZnSe are successively formed. On the light guiding layer 705, an active layer 706 of Zn.sub.0.8 Cd.sub.0.2 Se, a light guiding layer 707 of p-type ZnSe, a cladding layer 708 of a p-type ZnS.sub.0.07 Se.sub.0.93, a contact layer 709 of p.sup.+ -type ZnSe are formed. On the contact layer 709, an insulating layer 710 made of polyimide is formed. The insulating layer 710 has a contact hole 712. On the insulating layer 710, an electrode 711 of Au is formed. The electrode 711 and the contact layer 709 forms an ohmic contact via the contact hole 712. To the n-type GaAs substrate 701, an electrode 700 of Zn is connected so as to form an ohmic contact. By a current flowing from the electrode 711, holes and electrons are recombined in the active layer 706, so that blue-green light having a wavelength of 490 nm (77K) is emitted (see M. A. Haase et al., Appl. Phys. Lett. 59 (1991) 1272).
In a light emitting device of semiconductor, a reduction of defects such as dislocation is one of critical problems to be solved. Such defects as dislocation may serve as a non-radiative center or a recombination center of a deep level during the recombination of holes and electrons, which results in a reduction of the light-emitting efficiency. In addition, a local heat generation is caused, which may result in a destruction of a light emitting device or the like.
Since it is difficult to obtain a group II-VI semiconductor substrate having a large area and a high quality, a group II-VI compound semiconductor single crystal thin film is usually grown on a group III-V semiconductor substrate. Such a hetero-epitaxial growth is likely to cause defects such as dislocation due to mismatches of lattice constants and thermal expansion coefficients between the group III-V semiconductor substrate and the group II-VI compound semiconductor single crystal thin film. Such dislocation may reach an uppermost layer of a semiconductor multi-layer formed on the II-V semiconductor substrate unless the dislocation is combined with another dislocation.
In the case where the group II-VI compound semiconductor single-crystal thin film is to be grown by MBE, group II elements and group VI elements arrive at the single-crystal substrate. Among these elements, the group VI elements arrive at the substrate as polyatomic molecules. In order to decompose the molecules, a high-temperature growth is preferred. However, since the group VI elements has a high vapor pressure, the revaporization of the group VI atoms occurs at an extremely high rate on the single crystal substrate. Therefore, it is difficult to grow a group II-VI compound semiconductor thin film at a high temperature, and a point defect may easily be introduced. Accordingly, the growth of the group II-VI compound semiconductor thin film is generally performed at low substrate temperatures of about 260.degree. C. to 300.degree. C.
As a result, the decomposition of the group VI molecules (e.g., S.sub.8 .fwdarw.S.sub.2) and the incorporation of the group VI atoms generated by the decomposition into crystals do not smoothly advance, which results in a facet growth in an early growth stage. Therefore, a lot of defects such as dislocation, stacking defects are caused, and such structural defects may affect the overlying layer.
For the above reasons, in the group II-VI compound semiconductor thin film, it is difficult to fabricate a high-quality single-crystal thin film such as Si, GaAs, and an even super-thin film in an order of several atomic layers. As a result, a light emitting diode, a laser diode, or the like which utilizes such a group II-VI compound semiconductor thin film is of a low quality and lacks for stability. Specifically, it is impossible to obtain a laser which can stably and continuously oscillate at room temperature. It is only possible to obtain a laser which oscillates at low temperatures and has very short life time.
In order to obtain a group II-VI compound semiconductor with little dislocation, there generally exist two methods for reducing dislocation in a lattice mismatch system. In one method, the thickness of the layer is further reduced, and a strain is included in the thin film, so as to prevent the dislocation from occurring. In the other method, the dislocation which is once introduced is caused to escape in a direction perpendicular to the stacking direction, by using a strain superlattice.
As to the group II-VI compound semiconductor materials, in the conventional semiconductor laser device 720 shown in FIG. 16, the dislocation is prevented from occurring by making the thickness of the active layer 706 to be 100 angstroms or less. At the interface between ZnSe and ZnS.sub.x Se.sub.1-x (x=0.07), it is preferred that each of the light guiding layers 705 and 707 has a thickness of 100 angstroms or less. However, the light guiding layer 705 or 707 of ZnSe should have a function of confining light therebetween, that requires large thickness of the light guiding layer 705 or 707. As a result, in the semiconductor laser device 720 shown in FIG. 16, the introduction of the misfit dislocation cannot be avoided at an interface between the light guiding layer 705 or 707 of ZnSe and the cladding layer 704 or 708 of ZnS.sub.x Se.sub.1-x (x=0.07). Accordingly, a device destruction caused by defects occurs in the active layer 706, and the light guiding layers 705 and 707, which results in a short-term operation of the laser device at room temperature.
As described above, in order to obtain a group II-VI compound semiconductor thin film having a composition and a thickness suitable for effectively confining light in the prior art, the introduction of defects along with the lattice misfit is inevitable by material systems other than the MgZnSSe system which is a lattice match system. In addition, the thin film active layer having a strain exists at a position of a maximum light intensity distribution in the device structure, so that the active layer is easily deteriorated by light or heat generation. Accordingly, the device is significantly deteriorated during the high-temperature operation and the long-term operation.